Refractometers are commonly used to assess the maturity of fruit before harvest, to control processing in the food and beverage industries, and to check any industrially or clinically useful liquid for deterioration, adulteration, or error in preparation.
Most industrial refractometers are based on the measurement of the Critical Angle of Total Reflection and this invention applies to the most commonly used types which are read visually and do not depend on electronic devices to produce an output automatically.
Since the theory of critical angle refractometers may be found in textbooks on optics as well as in the patent literature (e.g. U.S. Pat. No. 3,267,795), it will be sufficient here to repeat that the measurement is carried out by reading the position of the edge of a dark area which appears in the field of view of the instrument. This edge is often called the refractometer Shadow Line and its exact position in the field of view is a function of the refractive index of the substance being tested. It may be read by either of two methods: